I believe that it should exist by default, can't immagine why it's not there. In any case in my proftpd.conf this voice is just under the setting for Anonymous access ...something like this :

bla bla
bla bla

&amp;/Anonymous&amp;

UseReverseDNS off

bla bla
bla bla

note : between the word UseReverseDNS and the word off there are 16 spaces, don't know if this has any importance, but I'm saying it in case it is important , just trying to help, forgive my ignorance.